Size press coating of paper substrates generally involves the application of a coating via a coater head onto the surface of the size press application roll and then transferring the coating from the size press application roll surface to the paper web or the like in a size press application nip. The paper web does not pick up all of the coating available in the application nip of the size press and thus, some of the coating is carried on the application roll surface back to the coater head where it is mixed with and picks up fresh coating delivered to the head and carries same through a metering nip formed at the outlet end of the coater head.
To facilitate uniform pick-up, it is essential that the wet film of coating on the roll surface between the coater head and the application nip of the size press be relatively uniform, hence, the use of some form of metering device generally a smooth metering rod at the outlet end of the coater head is used to form a metering nip with the size press roll. This rod is intended to ensure that the wet film on the roll between the metering nip and the application nip is as uniform as possible and to meter the amount of coating in the wet film of the roll, i.e. the thickness of the wet film measured from the roll surface.
The speed of the equipment (peripheral speed of the size press rolls) is relatively fast, generally over a thousand meters a minute. Thus, the hydrodynamic conditions generated in the coater head influences the ability of the metering rod to properly meter the coating. Another significant factor is the properties of the coating fluid as it approaches the metering nip.
Many different coater head designs have been suggested. However, the problems associated with the hydraulic conditions generated and the properties of the coating fluid particularly at the metering nip have persisted.
U.S. Pat. Nos. 4,396,648 issued Aug. 2, 1983 to Holt et al. (Consolidated Papers Inc), 4,405,661 issued Sep. 20, 1983 to Alheid (Beloit Corporation), 4,688,516 issued Aug. 25, 1987 to Sommer (Jagenberg A G), 4,839,201 issued Jun. 13, 1989 to Rantanen et al. (Valmet Paper Machinery Inc) and 4,945,855 issued Aug. 7, 1990 to Eklund et al. (Valmet Paper Machinery Inc) illustrate various different types of coater heads for application of coatings either directly to the paper or onto a size press roll for application in a size press applicator.
It is apparent that the flow passage geometries in these devices are all quite different but all generally have in common (with the exception of U.S. Pat. No. 4,945,855), a fluid delivery system that delivers the coating fluid to a chamber immediately preceding the metering device and simply permits the fluid to flow through the metering device in the direction of movement of the surface to which the coating is being applied. No particular precautions are taken or instructions given with respect to generating stable conditions in the coating fluid approaching the metering nip i.e. controlling the hydraulic conditions to avoid undue metering problems at the metering nip nor is any reference made to or teachings provided for adjusting the rheology of the coating to present a coating with the desired properties to the metering nip to facilitate its application.
U.S. Pat. No. 4,945,855 uses a different technique in that the fluid is applied before a first metering device and then flows past the first metering device to the final metering device and a return flow passage is provided from adjacent final metering device for its recirculation or removal of excess coating. As with the other devices, no accommodation is made to control vortexes that are formed upstream of the metering device.
U.S. Pat. No. 4,780,336 issued Oct. 25, 1988 to Damrau (Consolidated Papers Inc) does discuss the formation of turbulence and flow patterns in a high speed machine immediately upstream of the metering device (in this case a metering blade). This patent deems the added turbulence generated upstream of the metering nip to be uncontrollable i.e. the hydraulic conditions in the coating fluid approaching the metering nip are uncontrollable and teaches reinforcing the metering blade to reduce flexure caused by these uncontrolled conditions to a minimum and thereby improve the uniformity of the coating in spite of the uncontrollable turbulence.
U.S. Pat. No. 5,078,081 issued Jan. 7, 1992 to Kustermann (J. M. Voith gmbH) discloses a coating applicator head having a throttle gap leading into or toward the metering nip of the applicator and a second throttling channel upstream of the throttle leading to the metering end of the flow chamber, the second throttling channel permitting escape of excess coating fluid being delivered to the metering device. Apparently, the two throttles co-operate to adjust the flow to the metering end of the flow passage.